1. Field of the Invention
The invention relates to a fuel injection amount control system and a fuel injection amount control device for a multi-cylinder internal combustion engine.
2. Description of Related Art
Generally, an air-fuel ratio control system including a three-way catalyst disposed in an exhaust passage of a multi-cylinder internal combustion engine and an upstream air-fuel ratio sensor located upstream of the three-way catalyst has been widely known.
The air-fuel ratio control system is configured to calculate an air-fuel ratio feedback amount (main feedback amount) based on an output value of the upstream air-fuel ratio sensor, so that the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine (the air-fuel ratio of the engine, accordingly, the air-fuel ratio of exhaust gas) coincides with a target air-fuel ratio, and performs feedback control on the air-fuel ratio of the engine, using the main feedback amount. The feedback amount is a controlled variable common to all of the cylinders. The target air-fuel ratio is set to a given reference air-fuel ratio within the window of the three-way catalyst. Generally, the reference air-fuel ratio is the stoichiometric air-fuel ratio. The reference air-fuel ratio may be changed to a value in the vicinity of the stoichiometric air-fuel ratio, according to the intake air amount of the engine, the degree of degradation of the three-way catalyst, and so forth.
Generally, the air-fuel ratio control system as described above is applied to an internal combustion engine that employs an electronically controlled fuel injection system. The internal combustion engine has at least one fuel injection valve for each cylinder or an intake port that communicates with each cylinder. With this arrangement, if a fuel injection valve of a particular cylinder turns to “a characteristic that it injects fuel in an amount excessively larger than a designated fuel injection amount”, only the air-fuel ratio of the air-fuel mixture supplied to the particular cylinder (the air-fuel ratio of the particular cylinder) changes largely into a richer (smaller) value. Namely, the degree of ununiformity in the air-fuel ratio among cylinders (variations in the air-fuel ratio among cylinders, cylinder-to-cylinder air-fuel ratio imbalance proportion) becomes larger. In other words, significant imbalances appear among “the air-fuel ratios of the individual cylinders” as the air-fuel ratios of air-fuel mixtures supplied to the respective cylinders.
In the following description, the cylinder corresponding to “the fuel injection valve having a characteristic of injecting fuel in an amount excessively larger or excessively smaller than the designated fuel injection amount” will be called “imbalance cylinder”, and the remaining cylinders (cylinders corresponding to “fuel injection valves that inject fuel in the designated fuel injection amount”) will be called “non-imbalance cylinders (or normal cylinders)”.
If the fuel injection valve of a certain particular cylinder turns to “a characteristic that it injects fuel in an amount excessively larger than the designated fuel injection amount”, the average of the air-fuel ratios of air-fuel mixtures supplied to the engine as a whole becomes a richer air-fuel ratio than the target air-fuel ratio set to the reference air-fuel ratio. Accordingly, with the feedback amount of the air-fuel ratio common to all the cylinders, the air-fuel ratio of the particular cylinder is changed to a leaner (or larger) value so as to be closer to the reference air-fuel ratio, and at the same time, the air-fuel ratio of the other cylinders is changed to a leaner (or larger) value, deviate further from the reference air-fuel ratio. As a result, the average of the air-fuel ratios of the mixtures supplied to the engine as a whole (the average air-fuel ratio of exhaust gas) becomes equal to an air-fuel ratio in the vicinity of the reference air-fuel ratio.
However, the air-fuel ratio of the above-indicated particular cylinder is still a richer air-fuel ratio than the reference air-fuel ratio, and the air-fuel ratio of the remaining cylinders becomes a leaner air-fuel ratio than the reference air-fuel ratio. As a result, the amount of emissions discharged from each cylinder (the amount of unburned substances and/or the amount of nitrogen oxides) is increased, as compared with the case where the air-fuel ratio of each cylinder is equal to the reference air-fuel ratio. Therefore, even when the average of the air-fuel ratios of the mixtures supplied to the engine is equal to the reference air-fuel ratio, the increased emissions cannot be completely cleaned by the three-way catalyst, which may result in deterioration of the emissions.
In order to prevent emissions from being deteriorated, therefore, it is important to detect excessively large ununiformity in the air-fuel ratio among the individual cylinders (namely, the occurrence of air-fuel ratio imbalances among the cylinders), and take some measure against the imbalance condition. The air-fuel ratio imbalances among cylinders also occur, for example, when the fuel injection valve of a particular cylinder turns to “a characteristic that it injects fuel in an amount that is excessively smaller than the designated fuel injection amount”.
One example of fuel injection amount control system according to the related art obtains a trace length of an output value (output signal) of an electromotive force type oxygen concentration sensor located upstream of a three-way catalyst. The control system compares the trace length with “a reference value that varies according to the engine speed”, and determines whether an air-fuel ratio imbalance condition among cylinders has occurred based on the result of comparison (see, for example, U.S. Pat. No. 7,152,594).
In the meantime, if there is ununiformity or imbalances in the air-fuel ratio among the individual cylinders, the true average air-fuel ratio of the engine is controlled to “an air-fuel ratio that is larger than the reference air-fuel ratio (air-fuel ratio that is leaner than the reference air-fuel ratio)”, through main feedback control for making the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor equal to “the target air-fuel ratio set to the reference air-fuel ratio, such as the stoichiometric air-fuel ratio”. The reason will be hereinafter explained.
Fuel supplied to the engine is a compound of carbon and hydrogen. Accordingly, if the air-fuel ratio of an air-fuel mixture to be subjected to combustion is richer than the stoichiometric air-fuel ratio, unburned substances, such as “hydrocarbon HC, carbon monoxide CO and hydrogen H2”, are produced as intermediate products. In this case, as the air-fuel ratio of the mixture subjected to combustion, which ratio is richer than the stoichiometric air-fuel ratio, deviates further than the stoichiometric air-fuel ratio, the probability that the intermediate products meet and combine with oxygen during a combustion period is rapidly reduced. As a result, the amount of the unburned substances (HC, CO and H2) rapidly increases (for example, quadratically), as the air-fuel ratio of the mixture supplied to each cylinder becomes richer, as shown in FIG. 2.
Suppose that only the air-fuel ratio of a particular cylinder shifts to be largely richer than the stoichiometric ratio, causing “ununiformity (or imbalance) in the air-fuel ratio among the individual cylinders”. In this case, the air-fuel ratio of an air-fuel mixture supplied to the particular cylinder (the air-fuel ratio of the particular cylinder) changes to a greatly richer (or smaller) air-fuel ratio, as compared with the air-fuel ratio of air-fuel mixtures supplied to the remaining cylinders (the air-fuel ratio of the remaining cylinders). At this time, an extremely large amount of unburned substances (HC, CO, H2) are discharged from the particular cylinder. Accordingly, even if the average air-fuel ratio of the mixtures supplied to the engine is equal to “a certain specified value”, the total amount of hydrogen emitted from the engine when the degree of ununiformity in the air-fuel ratio among cylinders is large is significantly larger than the total amount of hydrogen that arises when there is no ununiformity (imbalance) in the air-fuel ratio among cylinders.
In the meantime, the upstream air-fuel ratio sensor has a porous layer (e.g., a diffusion resistance layer or protective layer) that permits gas (oxygen equilibrium gas) in a condition where unburned substances and oxygen are in chemical equilibrium to reach an air-fuel ratio sensing element. The upstream air-fuel ratio sensor generates a value (output value) commensurate with “the amount of oxygen (oxygen partial pressure, oxygen concentration) and the amount of unburned substances (partial pressure or concentration of unburned substances)” which have reached an exhaust-gas-side electrode layer (a surface of the air-fuel ratio sensing element) of the upstream air-fuel ratio sensor through the diffusion resistance layer.
On the other hand, molecules of hydrogen H2 are smaller in size than those of hydrocarbon HC and carbon monoxide CO, for example. Accordingly, hydrogen H2 diffuses into the porous layer of the upstream air-fuel ratio sensor more rapidly than the other unburned substances (HC, CO). Namely, selective diffusion (preferential diffusion) of hydrogen H2 takes place in the porous layer.
Accordingly, if the air-fuel ratios of the individual cylinders become unequal or non-uniform among the cylinders (if there arises ununiformity in the air-fuel ratio among the cylinders), the output value of the upstream air-fuel ratio sensor shifts to a richer value, due to the selective diffusion of hydrogen. Namely, the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor becomes “a richer air-fuel ratio” than the true air-fuel ratio of the engine. As a result, under the main feedback control, the true average air-fuel ratio of the engine is controlled to “an air-fuel ratio that is larger than the reference air-fuel ratio (an air-fuel ratio that is leaner than the reference air-fuel ratio)”.
On the other hand, exhaust gas that has passed through the three-way catalyst reaches a downstream air-fuel ratio sensor located downstream of the three-way catalyst. Hydrogen is converted and removed to some extent at the three-way catalyst. Accordingly, even when the degree of ununiformity in the air-fuel ratio among the cylinders is large, the downstream air-fuel ratio sensor generates an output value that is close to the true average air-fuel ratio of the engine.
Thus, another example of fuel injection amount control system according to the related art is configured to determine whether the degree of ununiformity in the air-fuel ratio among the cylinders is large, based on a parameter representing a degree of deviation between the air-fuel ratio sensed based on the upstream air-fuel ratio sensor and the air-fuel ratio sensed based on the downstream air-fuel ratio sensor (see Japanese Patent Application Publication No. 2009-30455 (JP-A-2009-30455).
The “shift of the air-fuel ratio to a learner (or larger) value due to selective diffusion of hydrogen and main feedback control” as described above will be simply called “erroneous lean correction”. The “erroneous lean correction” also occurs in the case where the air-fuel ratio of the imbalance cylinder is shifted to be leaner than the air-fuel ratio of the non-imbalance cylinders. Furthermore, the amount of shift of the air-fuel ratio to the lean side due to the erroneous lean correction increases as the degree of selective diffusion of hydrogen increases, and therefore increases as the degree of ununiformity in the air-fuel ratio among cylinders increases.
If the erroneous lean correction occurs, the true average air-fuel ratio of the engine (accordingly, the average of the true air-fuel ratio of exhaust gas) may become leaner (larger) than “the window of the three-way catalyst”. Accordingly, the NOx (nitrogen oxides) conversion efficiency of the three-way catalyst may be reduced, and the amount of NOx emissions may be increased.
As described above, the downstream air-fuel ratio sensor generates an output value that is close to the true average air-fuel ratio of the engine, even when the degree of ununiformity in the air-fuel ratio among the cylinders is large. Accordingly, the erroneous lean correction can be avoided if “known sub-feedback control” for making the output value of the downstream air-fuel ratio sensor equal to “a downstream-side target value corresponding to an air-fuel ratio around the stoichiometric ratio” is carried out.
However, the sub-feedback amount is often provided with the upper limit and the lower limit. If the sub-feedback amount becomes equal to the upper limit or the lower limit, the air-fuel ratio of the engine cannot be sufficiently controlled even with the sub-feedback amount, and the amount of NOx emissions may be increased. Also, the sub-feedback amount is adapted to change relatively slowly. Accordingly, even when the sub-feedback amount is not provided with the upper limit and the lower limit, or even when the sub-feedback amount does not coincide with the upper limit or lower limit, the amount of NOx emissions may be increased during a period, for example, after starting of the engine, in which period the sub-feedback amount is set to an inappropriate value.
To cope with the above-described situation, it is proposed to shift the air-fuel ratio of the engine to a richer air-fuel ratio (and consequently, to an air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio) when the degree of ununiformity in the air-fuel ratio among the cylinders becomes large. More specifically, the control system obtains an air-fuel ratio imbalance index value that increases as the degree of ununiformity in the air-fuel ratio among the cylinders increases, based on at least a value correlated with the output value of the upstream air-fuel ratio sensor. The air-fuel ratio imbalance index value, which has not been subjected to correction that will be described later, is also called “pre-correction air-fuel ratio imbalance index value” for the sake of convenience. Further, the control system controls the designated fuel injection amount so that the air-fuel ratio of the engine shifts to a richer air-fuel ratio as the pre-correction air-fuel ratio imbalance index value increases. Namely, the control system increases the designated fuel injection amount, so that “a designated air-fuel ratio (=the in-cylinder intake air amount/the designated fuel injection amount) as an air-fuel ratio determined by the designated fuel injection amount” becomes “a richer (smaller) air-fuel ratio” as the pre-correction air-fuel ratio imbalance index value increases. In this manner, it is possible to make up for the erroneous lean correction
However, if “the designated air-fuel ratio” is set based on the pre-correction air-fuel ratio imbalance index value, the amount of increase of the designated fuel injection amount may be excessively small or excessively large, and, in some cases, the emissions cannot be kept in a good condition. Having studied the phenomena, the inventor found that “even when the degree of ununiformity in the air-fuel ratio among the cylinders is equal to a particular value, the pre-correction air-fuel ratio imbalance index value may not necessarily be a constant value if the intake air amount Ga over a period in which the pre-correction air-fuel ratio imbalance index value is obtained (index value acquisition period) is different”. The reason that the pre-correction air-fuel ratio imbalance index value is influenced by the intake air amount Ga will be explained in detail later.
Even in the case where the “designated air-fuel ratio” is not set based on the pre-correction air-fuel ratio imbalance index value, if it is determined, based on the pre-correction air-fuel ratio imbalance index value, whether the ununiformity in the air-fuel ratio among the cylinders is excessively large (namely, whether an air-fuel ratio imbalance condition among cylinders has occurred), an erroneous determination may be made because the pre-correction air-fuel ratio imbalance index value is influenced by the intake air amount Ga over the index value acquisition period.